Transversally excited gas discharge lasers, especially transversally electrically excited lasers—TEA (Transverse Electrical discharge in gas at Atmospheric pressure) lasers—have been known since the mid-70s. These lasers stand out for a number of properties that are particularly important for various applications: very high laser output power with an adjustable magnitude and course, high beam quality, high operating safety, good controllability, compact construction and high quantum-optical efficiency. The gas discharge lasers have a broad application spectrum, among other things, as an excitation source in laser-induced, time-resolved spectroscopy, for intensity-dependent absorption and fluorescence examinations, as a test light source for electronic components and modules, as a source of ionization light in time-of-flight spectroscopy, for the opto-acoustic surface diagnostics of extremely thin layers, as a tool in micromaterial processing and for numerous applications under a UV microscope. The important aspect for many applications is especially the high pulse repetition rate with light pulses in the nanosecond and sub-nanosecond ranges.
The active laser cavity of a gas discharge laser consists of the height, the distance and the length of the main electrodes; this is where the stimulated light amplification takes place. The laser-active material is in the form of a gas or a gas mixture or vapor. The gas discharge can be excited optically or electrically. Popular gas lasers include CO2 lasers having a wavelength of 10.6 μm (far-infrared), for instance, for material processing, HeNe lasers having a wavelength of 633 nm (red), for example, for measuring technology, and excimer lasers having a wavelength ranging from 157 nm to 351 nm (ultraviolet, 157 nm F2, 31 nm XeF), for instance, for measuring technology and photochemistry. Mention should also be made of nitrogen lasers, which exhibit all of the prerequisites to become a significant and cost-effective UV source at 337.1 nm and at a bandwidth of 100 pm. The wavelength of 337.1 nm is ideal for many applications. Important methods that have been adopted worldwide are based on this wavelength. The active laser medium, nitrogen, does not require any laborious preparation for the operation of the laser. No chemical compounds are formed that would burden the environment. The physical characteristics of the laser allow the generation of very short light pulses ranging from a few nanoseconds to a few 100 ps without any complicated additional equipment. The use of excimer gas mixtures is likewise possible, also when halogen gases are involved.
The energy feed needed to amplify the light is obtained by means of a high-current discharge of 1000 to 3000 A cm−2 through the entire active laser cavity. The high-current discharge causes most of the gas molecules of the laser medium to be brought into a higher-energy excited state. A randomly occurring photon of the same direction and wavelength or a fed-in oscillator photon then starts the amplification process. The light wave runs between the two resonator mirrors until most of the gas molecules are no longer in the laser-active excited state. Part of the light energy is outcoupled via the output coupler and is then available as a monochromatic, oriented light pulse. The high-current discharge is fed from low-inductivity capacitors in which the requisite electric energy is temporarily stored during the time span between the laser pulses. The temporarily stored energy is conducted into the laser cavity via a heavy-duty circuit-breaker at a rate of current rise ranging from 100 to 500 kAμ−1.
There are gas discharge lasers in various versions in terms of their essential components, namely, the laser channel and the excitation circuit. The laser channel is arranged in a discharge vessel in which the gas discharge path is located, and it usually consists of two main electrodes situated across from each other and having an overall length of 10 mm to 500 mm. There are gas discharge lasers with an external gas reservoir (German patent application DE 41 27 566 A1) as well as gas discharge lasers with a gas cavity that is sealed-off in the discharge vessel and that is cooled by passive or active circulation. In many gas discharge lasers, a first problem is encountered with the gas-tightness of the gas discharge vessel. German utility model DE 295 20 820 U1 describes a laser tube for gas discharge lasers containing halogen in which all of the openings are sealed off by interposed metal gaskets. German utility model DE 297 13 755 U1 describes a gas discharge laser with a ceramic gas discharge tube in which the metal gaskets have an elastic core. German patent application DE 40 10 149 A1 describes a high-frequency excited strip conductor laser in which the resonator mirror is connected galvanically directly to the main electrodes. German patent application DE 101 47 655 A1 describes a gas discharge laser in which an insulator is provided that largely encloses the gas discharge chamber and that is configured as a single piece in order to save on gaskets. German utility model DE 202 12 624 U1 describes a compact excimer laser for generating high pulse rates in which a main electrode is arranged in a ceramic discharge tube, with the main electrode forming the gas discharge path leading to several heat exchanger tubes. The front ends of the discharge tube are sealed off by two flanges, with interposed gaskets. In addition to a pre-ionizer, several current formers and a fan are arranged in the discharge tube. Several larger ionization capacitors are provided on the outside of the discharge vessel as part of the discharge circuit. German patent application De 36 19 354 A1 describes a transversally excited pulse gas laser that has radially adjustable main electrodes, which requires an asymmetrical construction.
In addition to the laser channel with the discharge vessel and the main electrodes, the electrical excitation circuit may also play a role in a gas discharge laser when it comes to achieving a high current discharge between the main electrodes. Generally, the electrical excitation circuit consists of at least two capacitors, the main electrodes, a fast switch, for example, a spark gap or thyratron, as well as ribbon cables for fast and low-loss energy transmission. German utility model G 80 25 354 U1 describes a high-energy laser of the TEA type with pre-ionization rods arranged parallel to the laser axis, whereby the pulse-forming electrical excitation circuit can be configured as a Blümlein circuit or as a charge-transfer circuit. With the Blümlein circuit, the high voltage is applied to a first charging capacitor via a low-inductivity switch. Parallel to this is the series connection of a second charging capacitor and the gas discharge path. The basic circuit diagram of the Blümlein circuit is shown in German patent application DE 44 08 041 A1 for a sub-nanosecond nitrogen laser. The charge-transfer circuit differs from the Blümlein circuit especially in that the second charging capacitor lies parallel to the gas discharge path and in that the first charging capacitor is connected in series to the low-inductivity switch and thus to the high-voltage supply. Therefore, with the charge-transfer circuit, only switching pulses of up to about 1 ns with a pulse rise time of the switch of about 40 ns are possible, in contrast to which the Blümlein circuit can generate pulses of up to 50 ps at a switch rise time of less than 12 ns. The product brochure titled “ALLTEC Power Switch” of the ALLTEC company describes the use of a semiconductor switch for operating the high-voltage discharge of TEA CO2 lasers. This can replace conventional thyristors but still has similarly large dimensions within the range of a few 100 mm.
In order to obtain a laser discharge with a uniform volume while avoiding the formation of an electric arc at high gas pressures, it is advantageous to attain an initial concentration of ions and electrons in the gas discharge path prior to applying the electric main pulse. This technique is referred to as pre-ionization and is described in German patent application 28 11 198 A1. One way to achieve a pre-ionization includes using the characteristic Blümlein voltage pre-pulse that is formed at the main electrodes during the charging cycle of the Blümlein transmission line. The amplitude of this fast pulse is sufficiently large so that an initial disturbance of electrons in the vicinity of the main electrodes is achieved. It is, however, also sufficiently short so that a complete breakdown of the laser gas is prevented. The magnitude and time behavior of the pre-pulse are controlled by the pulse-charging rate of the Blümlein transmission line and can be changed over a wide range of values so that optimal pre-ionization is achieved. The high-current discharge is applied directly to the auxiliary electrodes in order to achieve a pre-ionization and via the contact electrodes on the outside of the discharge vessel on the main electrodes. German patent application DE 37 34 690 A1 describes a procedure to use a contact electrode on the outside of the discharge vessel as a capacitor electrode.
DE 33 13 811 A1 is a gas discharge laser with the above-mentioned components that has a cylindrical, gas-tight (sealed-off) discharge vessel, for instance, made of ceramic aluminum oxide Al2O3 that is sealed off at both ends by resonator mirrors, with interposed gaskets. Between each pair of electrodes, the pre-ionization is brought about outside of the gas discharge path. The main electrodes are mushroom-shaped and are secured by means of decoupled supports and feed lines that pass through the vessel wall and are connected to the excitation circuit. The excitation circuit has, in addition to a capacitor connected in parallel to a gas discharge path, also a triggerable spark gap and is likewise connected to main and auxiliary electrodes. No statements are made about the adjustment possibilities of the main electrodes and of the resonator mirrors. Such a gas discharge laser, however, entails several drawbacks, as a result of which it is difficult to condition. These drawbacks include the fact that the static and dynamic service life of the gas is limited due to leaks that occur at the joints on the discharge vessel and due to the relatively high diffusion rates of the individual components. Moreover, since these lasers typically cannot be heated to temperatures very much above 100° C., it becomes very time consuming (approximately 24 hours) to create the requisite hygienic conditions for the gas in the discharge vessel, especially with respect to the dipolar water residues that inevitably lead to aggressive compounds containing laser-gas components that can render the laser non-operational, in addition to which the gas has to be purged several times. The switch in the excitation circuit and the discharge channel are modules that are subject to wear and tear. A standby power source has to be provided. The external construction elements give rise to losses and the need for additional resources. Finally, the gas discharge laser displays a time jitter, a temperature gradient and especially a tolerance zone for the optical interface owing to adjustment inaccuracies during the production of the laser and to errors in the readjustment possibilities during operation.